The erythrocyte sedimentation rate (ESR) is a widely used test which can help determine if a patient has a condition which is causing sub-acute or chronic inflammation. It Is non-specific, but It is still very helpful, particularly in following the course of some inflammatory diseases. Some of the problems with the test are that it uses a relatively large amount of blood (about one-half ml or more); it normally requires at least sixty minutes to complete, and thus is somewhat impractical for immediate patient care; and it requires the operator to handle bloody equipment. Additionally, the test requires a relatively high degree of skill and care in setting it up if the results are to be trusted.
The Westergren ESR test, which is the "Gold Standard" reference method for the ESR, is performed by placing a diluted sample of anticoagulated blood in a tall, perfectly vertical tube and measuring how far the plasma/erythrocyte (P/E) interface has settled under the influence of gravity after sixty minutes. The test works because the proteins associated with inflammation, particularly fibrinogen, counteract the zeta potential of red blood cells, which is a negative surface charge on the erythrocytes that serves to repel the individual erythrocytes from each other and thus prolong erythrocyte sedimentation. When systemic inflammation is present, the fibrinogen content of the blood increases, and the erythrocytes tend to aggregate, and thereby decrease their surface-to-mass ratio, and thus increase their rate of sedimentation.
Various approaches to automation have been attempted, notably the use of electronic means to track the sedimentation of the erythrocytes and provide a result in less than the usual sixty minutes, but these methods require at least fifteen minutes, and their results do not correlate well with the "reference" ESR method, i.e., that of Westergren.
A centrifugal method for packing blood so as to determine the approximate ESR was invented by Brian S. Bull et al, and is described in U.S. Pat. No. 3,824,841. The Bull et al method is, however, undesirable since it requires a large amount of blood and subjects the clinician performing the test to the possibility of exposure to infectious aerosols.
Another centrifugal method for measuring the ESR has been proposed by James W. Winkelman et al and is described in International Patent Application No. WO 96/39618. The Winkelman et al technique solves many of the problems of previous ESR techniques in that it uses a smaller quantity of blood and provides results within a few minutes. The Winkelman et al method involves the centrifugation of a quantity of blood in a disc-shaped rotor chamber and the observance of position of the P/E interface as the blood cells pack under centrifugal force. The total time from the start of centrifugation to the cessation of movement of the interface is related to the ESR. This device also requires a modest amount of blood, and the results moderately correlate with the Westergren method.
It would be desirable to be able to perform a rapid ESR measurement using a conventional blood sample container, such as a capillary tube, or the like, so as to derive an ESR from a small amount of blood in a matter of minutes